The spore-forming microorganism Bacillus thuringiensis (B.t.) produces the best-known insect toxin. The toxin is a protein, designated as .delta.-endotoxin. It is synthesized by the B.t. sporulating cell. The toxin, upon being ingested in its crystalline form by susceptible insect larvae, is transformed into biologically active moieties by the insect gut juice proteases. The primary target is insect cells of the gut epithelium, which are rapidly destroyed. Experience has shown that the activity of the B.t. toxin is so high that only nanogram amounts are required to kill susceptible insects.
The reported activity spectrum of B.t. covers insect species within the order Lepidoptera, which is a major insect problem in agriculture and forestry. The activity spectrum also includes the insect order Diptera, wherein reside mosquitoes and blackflies. See de Barjac, H. ([1990] In H. de Barjac, D. J. Sutherland (eds.) Bacterial Control of Mosquitoes and Blackflies, Rutgers University Press, Chapter 2). U.S. Pat. Nos. 4,771,131 and 4,996,155 disclose toxin genes which are active against beetles of the order Coleoptera. Activity has also been reported outside the class Insecta. B.t. strains having activity against nematodes were disclosed in U.S. Pat. No. 4,948,734.
Aphids (Order Hemiptera, Family Aphididae) are sucking insects that are damaging to many economically important plants. Plant damage occurs when aphids infest plants in high numbers. Because aphids can reproduce by parthenogenesis, aphid populations have the potential to increase rapidly. High numbers of aphids can contribute to fungal infestations. In addition to the damage caused by aphid feeding, aphids can vector many viral diseases of plants.
Economically important aphids include the green peach aphid (Myzus persicae), pea aphid (Acyrthosiphon pisum), cabbage aphid (Brevicoryne brassicae), cotton aphid (Aphis gossypii), and black bean aphid (Aphis fabae).